1. Field of the Invention
The present invention relates to silicon-containing particles suitable for a negative electrode active material for a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
In recent years, as portable electronic devices and communication devices develop remarkably, from the viewpoint of economic efficiency and miniaturization and light weight of devices, a nonaqueous electrolyte secondary battery having a high energy density is in strong demand.
Since silicon has a theoretical capacity of 4200 mAh/g far higher than 372 mAh/g of a theoretical capacity of a carbon material that is being put into practical use at the present time, silicon is a material that is most highly expected in attaining miniaturization and higher capacity of a battery.
For example, in Patent Document 1, a lithium ion secondary battery where single crystal silicon is used as a base material of a negative electrode active material is disclosed.
Further, in Patent Document 2, lithium ion secondary batteries each of which uses a lithium alloy of LixSi (x is 0 to 5) of single crystal silicon, polycrystalline silicon or amorphous silicon are disclosed. In particular, LixSi that uses amorphous silicon is preferred, and a crushed material of crystalline silicon covered with amorphous silicon obtained by plasma decomposition of monosilane is exemplified.
However, in this case, as shown in example, since 30 parts by mass of silicon and 55 parts by mass of graphite as a conductive agent are used, a battery capacity of silicon could not be fully exerted.
Further, in Patent Documents 3 to 5, a method of utilizing a negative electrode that was obtained by vapor depositing an amorphous silicon thin film on an electrode current collector is disclosed.
In the method of vapor depositing silicon directly on a current collector, also a method where, by controlling a growth direction, the cycle characteristics is prevented from deteriorating due to volume expansion is disclosed (see Patent Document 6). According to the method, it is said that a negative electrode having a high capacity and excellent cycle characteristics can be obtained. However, there were problems that since the production speed is limited, coat is high for the reason why the production speed is limited, a silicon thin film is difficult to make thicker, and copper that is a negative electrode current collector diffuses in silicon.
Therefore, in recent years, a method where while using silicon-containing particles, by limiting a utilization rate of battery capacity of silicon, a volume expansion is suppressed (see Patent Documents 7 to 9), a method where as a method of using a grain boundary of polycrystalline particles as a buffer zone of volume change, a silicon melt to which alumina is added is quenched (see Patent Document 10), a method where polycrystalline particles of mixed polycrystals of α, β-FeSi2 are used (see Patent Document 11), and a method where a single crystal silicon ingot is processed by high temperature plastic working (see Patent Document 12) are disclosed.
As was described above, in order to utilize silicon as an active material, metallic silicon and silicon alloys, which have various crystal structures have been proposed. However, all of these are disadvantageous from the viewpoint of cost, that is, a method that allows to cheaply mass-produce has not been proposed.
Patent Document 1: Japanese Patent No. 2964732
Patent Document 2: Japanese Patent No. 3079343
Patent Document 3: Japanese Patent No. 3702223
Patent Document 4: Japanese Patent No. 3702224
Patent Document 5: Japanese Patent No. 4183488
Patent Document 6: Japanese Unexamined Patent Application Publication No. 2006-338996
Patent Document 7: Japanese Unexamined Patent Application Publication No. 2000-173596
Patent Document 8: Japanese Patent No. 3291260
Patent Document 9: Japanese Unexamined Patent Application Publication No. 2005-317309
Patent Document 10: Japanese Unexamined Patent Application Publication No. 2003-109590
Patent Document 11: Japanese Unexamined Patent Application Publication No. 2004-185991
Patent Document 12: Japanese Unexamined Patent Application Publication No. 2004-303593